Coordinated trafficking of synaptic vesicle and active zone proteins prior to synapse formation

Here, using time-lapse confocal imaging of the dynamics of PTVs and STVs in the same axon, we show that vesicle trafficking is coordinated through at least two mechanisms. First, a significant proportion of STVs and PTVs are transported together before forming a stable terminal. Second, individual PTVs and STVs share pause sites within the axon. Importantly, for both STVs and PTVs, encountering the other type of vesicle increases their propensity to pause. To determine if PTV-STV interactions are important for pausing, PTV density was reduced in axons by expression of a dominant negative construct corresponding to the syntaxin binding domain of syntabulin, which links PTVs with their KIF5B motor. This reduction in PTVs had a minimal effect on STV pausing and movement, suggesting that an interaction between STVs and PTVs is not responsible for enhancing STV pausing.Our results indicate that trafficking of STVs and PTVs is coordinated even prior to synapse development. This novel coordination of transport and pausing might provide mechanisms through which all of the components of a presynaptic terminal can be rapidly accumulated at sites of synapse formation.In the cortex, the bulk of synapse formation occurs at high rates over a period of several weeks during early postnatal development. Formation of individual synapses is triggered when axons and dendrites contact one another [1,2]. For these axo-dendritic contacts to stabilize and form a synapse, synaptic vesicle and active zone proteins must be recruited to the site of contact very rapidly, within minutes to hours [3-10]. This rapid assembly is remarkable, considering that each presynaptic protein needs to be transported from the soma to individual sites of synapse formation along the axon.Recent work using live imaging of developing neurons has revealed mechanisms that might facilitate rapid synapse assembly. For example, rather than transport each molecule individually, neurons package multiple presynaptic compo